(CNN) -- Sen. John McCain has decided on his running mate and will inform the person Thursday night, sources close to the presumptive Republican presidential nominee said.

Sen. John McCain reportedly is set to announce his vice presidential pick Friday in Ohio.

A Republican source said the matter was settled at a meeting of McCain's advisers Wednesday.

Plans are in place for the senator from Arizona to reveal his pick for the GOP vice presidential nominee at an Ohio rally Friday, the day after Sen. Barack Obama formally accepts the Democratic presidential nomination.

The McCain campaign is hoping to have 15,000 people at the Ohio event, roughly five times the size of his largest crowd to date.

In an interview with a Pennsylvania radio station that was taped Wednesday and aired Thursday, McCain said he had not settled on a nominee. The Associated Press reported that the interview with KDKA radio had occurred Thursday, which created confusion as to whether McCain had finalized his choice for vice president. Watch who is on McCain's short list

A Republican insider said this month that McCain campaign manager Rick Davis has called several state party chairmen and indicated that Ridge will be the Republican vice presidential pick this cycle.

During his interview with KDKA, McCain praised Ridge, saying, "he's a great American and a great and dear friend, and I rely on him, and I have for many years."

But the possibility that Ridge could be McCain's running mate sparked a backlash among conservatives because he supports abortion rights.

Another potential VP pick, Sen. Joseph Lieberman of Connecticut, also raised concerns among conservatives. Lieberman, an independent senator who was the 2000 Democratic vice presidential nominee, has been a vocal supporter of the war in Iraq, but he, too, backs abortion rights.

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If McCain picks either man, it could drive away social conservatives who are already uneasy about his nomination, conservative activists warned.

In an open letter to McCain, conservative activist Richard A. Viguerie wrote, "Your indication that you're willing to put a person who has a clear, unequivocal pro-abortion record within a heartbeat of the presidency is alarming."

"Sen. McCain, you are exceedingly proud of being a political maverick -- you wear it as a badge of honor. Well, poke the base of the Republican Party -- the conservatives -- in the eye one more time by choosing a pro-abortion vice presidential candidate, and conservatives will show you that two can play the maverick game," Viguerie said. iReport.com: Your pick for VP

Minnesota Gov. Tim Pawlenty is also believed to be on McCain's short list, as is Romney, who ran against McCain this year for the GOP presidential nomination and was a frequent critic of the senator on the campaign trail. Watch Pawlenty discuss his VP possibilities »

But Romney endorsed McCain after he captured the Republican nomination and has campaigned for him. He also opposes abortion rights, a position he said he came to in 2004 after studying the stem cell-issue as Massachusetts governor. Before then, Romney was in favor of abortion rights.